Loadbreak connectors used in conjunction with 15 and 25 KV switchgear generally include a power cable elbow connector having one end adapted for receiving a power cable and another end adapted for receiving a loadbreak bushing insert. The end adapted for receiving the bushing insert generally includes an elbow cuff for providing an interference fit with a molded flange on the bushing insert. This interference fit between the elbow cuff and the bushing insert provides a moisture and dust seal therebetween. An indicator band may be provided on a portion of the loadbreak bushing insert so that an inspector can quickly visually determine proper assembly of the elbow cuff and the bushing insert.
The elbow cuff forms a cavity having a volume of air which is expelled upon insertion of the bushing insert. During initial movement of the loadbreak connectors in the disassembly operation, the volume of air in the elbow cavity increases but is sealed off at the elbow cuff resulting in a decrease in pressure within the cavity. The dielectric strength of the air in the cavity decreases with the decrease in air pressure. Although this is a transient condition, it occurs at a critical point in the disassembly operation and can result in dielectric breakdown of the opening interface causing a flashover or arc to ground. The occurrence of flashover is also related to other parameters such as ambient temperature, the time relationship between the physical separation of the connectors and the sinusoidal voltage through the loadbreak connectors.
Another reason for flashover while switching loadbreak connectors prior to contact separation is attributed to a decrease in dielectric strength of the air along the interface between the bushing insert and the power cable elbow to ground. As earlier described, a decrease in air pressure occurs momentarily in the sealed cavity between the elbow cuff and the bushing insert flange. The lower pressure in the cavity reduces the dielectric strength of the air along the connection interface, which can possibly result in flashover.
In the prior art 25 kV loadbreak connectors, the cuffs on the power cable elbow do not extend past the energized portion of the probe (see FIG. 2). Moreover, the conductive jacket on the bushing is not shielded by insulation (see FIG. 1). Consequently, the flashover distance was simply the distance (“D”) between the top of the bushing insert (i.e., the end of the bushing insert connected to a power cable elbow) to the conductive jacket around the mid-section of the bushing insert (see FIG. 3). There were no measures taken to reduce flashover by increasing the flashover distance. Prior art 35 kV loadbreak elbows have used extended elbow cuffs to increase the flashover distance.
Another problem encountered with prior art loadbreak connectors is that it is difficult to insert one end of the loadbreak bushing insert into the power elbow connector and the opposite end into a bushing well. In particular, because the interface surfaces of the loadbreak bushing insert and the power elbow connector are typically made from a rubber material, substantial frictional forces make it difficult to insert the loadbreak bushing insert into the power elbow, even when lubricated. When the loadbreak connector is assembled, the rubber to rubber surfaces tend to stick together before the elbow connector is properly seated on the bushing insert.
Accordingly, it would be advantageous to design a loadbreak connector system that includes a power cable elbow and a loadbreak bushing insert, which reduces or prevents the possibility of a flashover upon switching of the connectors. In addition, there is a need for a sleeve made from a low coefficient of friction material that reduces the friction between the power cable elbow and the bushing insert during connection and disconnection.